This invention relates to circuit breakers, and, more particularly, to a circuit breaker rotary contact arm arrangement.
U.S. Pat. No. 4,616,198 entitled CONTACT ARRANGEMENT FOR A CURRENT LIMITING CIRCUIT BREAKER describes the early use of a first and second pair of circuit breaker contacts arranged in series to substantially reduce the amount of current let-through upon the occurrence of an overcurrent condition.
When the contact pairs are arranged upon one movable rotary contact arm such as described within U.S. Pat. No. 4,910,485 entitled MULTIPLE CIRCUIT BREAKER WITH DOUBLE BREAK ROTARY CONTACT, some means must be provided to insure that the opposing contact pairs exhibit the same contact pressure to reduce contact wear and erosion.
One arrangement for providing uniform contact wear is described in U.S. Pat. No. 5,310,971 entitled ROTARY CONTACT SYSTEM FOR CIRCUIT BREAKERS. This arrangement includes a rotary contact arm that employs rollers between the movable contact arm and spring pins to reduce contact arm friction. A rotor assembly with four contact springs, two on each side of the rotor, offset from the center of the rotor to impart contact force between the fixed and movable contacts is also disclosed. However, the roller system used in this arrangement can cause friction between the rollers and contact arm, which will result in uneven contact forces and, therefore, uneven contact wear. In addition, a rotor with springs offset from the rotor's axis of rotation can cause a non-uniform force distribution between the fixed and movable contact pairs if one pair of contacts erodes more than the other pair. The erosion of the contact pair with lower force results in a further reduction in force that continues to accelerate the erosion process.